TFT LCD devices are generally classified into a reflective TFT LCD device using a reflective layer as pixel electrodes, a transmissive TFT LCD device using transparent pixel electrodes, and a transflective TFT LCD device using a reflective layer having a transmissive region in a portion of a reflective region as pixel electrodes, according to reflectance or permeability of pixel electrodes. In the TFT LCD devices, to supply voltage for controlling arrangement of liquid crystal to the pixel electrodes, drain electrodes of thin film transistors formed in each pixel are connected to the corresponding pixel electrodes. The pixel electrodes are generally connected to the drain electrodes through via holes formed in an interlayer insulating layer.
In a transmissive TFT LCD device, pixel electrodes use indium oxides to form transparent electrodes. However, this material may cause a problem that oxidizes wires of aluminum (Al) to form insulating oxides and thereby hinders in supplying voltage to the pixel electrodes. Therefore, in the transmissive TFT LCD device, drain electrodes are formed of a single layer of metal such as chromium (Cr), or a two-layered conductive layer having an Al-contained metal layer and a molybdenum tungsten (MoW) or Cr layer formed on the Al-contained metal layer.
In a reflective TFT LCD device, pixel electrodes usually use aluminum neodymium (AlNd). In this case, materials forming drain electrodes are also limited. Referring to FIG. 1, a pixel portion of the reflective TFT LCD device using an easily oxidized metal such as Al as source and drain electrodes 21, 21′ is illustrated. On the source and drain electrodes 21, 21′, a protecting layer 23, for example a photosensitive organic insulating layer is disposed. The protecting layer 23 has via holes for connecting the drain electrodes 21′ to the pixel electrodes 27. Therefore, in an exposure, development and cleaning process of photolithography for forming the via holes, if developer or detergent of strong oxidant contacts the drain electrodes 21′ through the via holes, the upper layer of the drain electrodes 21′ may form insulating oxides 25. The insulating oxides 25 increase contact resistance between the drain electrodes 21′ and pixel electrodes 27.
To solve the problem, an upper layer 212′ of the drain electrode 21′ can be formed of metal such as MoW that is resistant against oxidation, as shown in FIG. 2. However, in this case, battery effect, like inside a chemical battery, can be occurred due to difference of electro-negativity between the upper layer 212′ of the drain electrode 21′ and an Al-containing reflective layer forming the pixel electrodes 27. For example, due to corroding by the battery effect, gaps 29 similar to spike phenomenon generating at the interface between a silicon layer and an Al layer can be formed at the interface between the upper layer 212′ and the Al-containing reflective layer. Also, as a portion of the Al-containing reflective layer around the gaps 29 falls, the Al-containing reflective layer can generate cracks 31 around the via holes. These gaps 29 or cracks 31 cause a problem increasing contact resistance between the pixel electrodes 27 and the drain electrodes 21′.
Generally, the battery effect increases in proportion to the difference of surface area and electronegativity between two metal layers. Accordingly, the drain electrodes 21′ that usually has relatively very small surface area compared to the pixel electrodes 27 enforces the battery effect more, thereby increasing the contact resistance between the pixel electrodes 27 and the drain electrodes 21′ more.
To solve the battery effect, it can be considered to use a MoW or Cr layer as reflective plates or pixel electrodes 27. However, such a choice deteriorates reflectance and conductivity of the pixel electrodes.
Accordingly, a new TFT LCD device, which can prevent increase of contact resistance due to the battery effect or surface oxidation at the interface between the pixel electrodes and the drain electrodes with maintaining highly reflectance and conductivity, is required.